A substantial proportion of the elderly population will suffer fractures associated with low bone mass. [Ch. 4. The Frequency of Bone Disease. In: Bone Health and Osteoporosis: A Report of the Surgeon General. Rockville, Md.: U.S. Department of Health and Human Services, Office of the Surgeon General (2004)]. Ten million Americans 50 years of age and older already have osteoporosis and another 33 million have osteopenia, the total population with low bone mass is estimated to reach 61 million by 2020 [Khosla, et al., Journal of Bone and Mineral Research 26:2565 (2011)]. In 2005 almost 2 million osteoporosis-related fractures were reported. The number of such fractures is expected to exceed 3 million by 2025 [Khosla, et al., (2011)].
While osteopenia and osteoporosis are problems commonly associated with post menopausal women, they also occur in older men and in men suffering from low testosterone [Riggs, et al., Endocr. Rev. 23:279 (2002)]. Low bone mass is also a significant problem in those suffering from diseases of the gastrointestinal tract such as inflammatory bowel disease (IBD); including Crohn's disease, ulcerative colitis, pouchitis and microscopic colitis [reviewed in Ali, et al., Am. J. Med. 122:599 (2009)]. In addition, those that have undergone gastrectomy or small bowel resection, as well as patients undergoing corticosteroid therapy are at higher risk for osteopenia or osteoporosis [Coates, et al., J. Clin. Endocrinol. Metab. 89:1061 (2004); Ali, et al., (2009)].
The normal physiologic process of bone remodeling involves balancing rates of bone resorption and bone synthesis. Bone resorption is mediated by osteoclasts, whereas bone synthesis is largely carried out by osteoblasts. Typically, bone is maintained and damaged bone repaired by the coordinate actions of osteoclasts and osteoblasts. The first cell type facilitating resorption of damaged bone and the second synthesizing new bone. Osteopenia and osteoporosis occur when accelerated rates of bone resorption and declining rates of new bone synthesis alter the mineral content, density and structure of bone [Ch. 2. The Basics of Bone in Health and Disease. In: Bone Health and Osteoporosis: A Report of the Surgeon General (2004)].
A mouse model for post menopausal induced osteoporosis is available. This model involves removing the ovaries of young (approximately eight to twelve week old) mice to provoke estrogen deficiency. Within a few weeks of overiectomy, the mice exhibit significantly reduced bone mass and display all the morphological bone characteristics of osteoporosis, including highly increased incidence of low trauma fractures [Seidlova-Wuttke, et al., Comp. Med. 62:8 (2012)]. Another mouse model, directed at inducing IBD has also been found to produce osteopenia and osteoporosis. In this system, mice are treated with dextran sodium sulfate (DSS), usually by providing drinking water supplemented with a few percent DSS for a period of a few days or weeks [Hamdani, et al., Bone 43:945 (2008); Harris, et al., Am. J. Physiol. Gastrointest. Liver Physiol. 296:G1020 (2009)]. Mice that are exposed to DSS in drinking water will develop an inflammation of the colon, displaying symptoms such as diarrhea, rectal bleeding, and weight loss. Acute colitis can be induced by a single cycle of DSS exposure lasting only a few days; longer exposure or multiple cycles of exposure can result in chronic colitis.
These two mouse models provide the opportunity to examine the genesis and treatment of bone loss and bone disorders under two different physiologic conditions. The overiectomized mouse model allows investigation of osteopenia and osteoporosis in a system in which the chronic inflammation associated with IBD is absent, whereas the DSS system allows investigation of osteopenia and osteoporosis in which chronic inflammation of the gut, typical of IBD is the direct, though distal cause of bone loss. Both these animal models have proven to be useful tools for understanding the biological consequences of low bone density-related diseases and in the development of therapeutic and prophylactic treatments.
Hormone replacement therapy (HRT) was once the main treatment for osteopenia and osteoporosis in post-menopausal women, because it effectively rebalances bone resorption and new bone synthesis. Bisphosphonate compounds, such as risedronic acid (Actonel®), alendronic acid (Fosamax®), and pamidronic acid (Aredia®), have largely replaced HRT due to concerns that HRT may increase risk of cancer. Bisphosphonates conserve bone mineral density and reduce fracture risk by slowing the rate of bone resorption; however they do not stimulate new bone synthesis. Long term use of bisphosphonates may be harmful to overall bone health, since the normal bone remodeling process is inhibited and damaged bone is not efficiently repaired. For general discussion of the risks and benefits of bisphosphonates see National Osteoporosis Society “Drug Treatment” [National Osteoporosis Society. “Drug Treatment” Camerton, Bath BA2 OPJ United Kingdom, revised June 2012 (2012); Ch. 9. Prevention and Treatment for Those Who Have Bone Diseases. In: Bone Health and Osteoporosis: A Report of the Surgeon General (2004)].
Treatment of IBD frequently involves use of immunosuppressant glucocorticoids, which are known to adversely affect bone density [Long, et al., Dig. Dis. Sci. 55:2263 (2010)]. However, severe bone loss is apparent in untreated IBD sufferers and therefore is not a consequence of glucocorticoid treatment in these patients [Bjarnason, Rheumatology 38:801 (1999)]. Different kinds of IBD result in bone loss, indicating that the connection between the two pathologies is common to IBD in general and is not particular to Crohn's disease, ulcerative colitis, or any of the other specific conditions generally classified as IBD.
Interestingly, DSS treated mice appear to develop osteopenia by a reduction in bone formation, as opposed to an increase in bone resorption [Harris, et al., (2009)]. DSS induced IBD has been reported to not only reduce osteoblast activity but simultaneously increase the number of osteoclasts, most of which appear to be inactive [Harris, et al., (2009)]. These findings are supported by an alternative approach to assessing conditions associated with chronic systemic inflammation that does not involve DSS. In this case, knockout mice lacking anti-inflammatory IL-10 were also observed to exhibit osteopenia and osteoporosis, confirming that the bone loss in DSS-treated mice is general to chronic inflammation and therefore, not likely a specific consequences of DSS exposure via some other physiologic route [Dresner-Pollak, et al., Gastroenterology 127:792 (2009)].
The balance between bone resorption by osteoclasts and bone formation by osteoblasts is regulated by several proteins produced by osteoblastic cells [reviewed in Ghishan and Kiela, Am. J. Physiol. Gastrointest. Liver Physiol. 300:G191 (2010); also see Ch. 2. In: Bone Health and Osteoporosis: A Report of the Surgeon General (2004) especially FIG. 2-6]. Osteoblastic production of Macrophage Colony Stimulating Factor (M-CSF) and Receptor Activated Nuclear Factor Kappa B Ligand (RANKL) stimulates development and activity of osteoclasts. Osteoblasts also produce osteoprotegerin (OPG), which functions as a soluble decoy receptor for RANKL and the local ratio of RANKL to OPG likely modulates osteoclast activity at sites of new bone synthesis. Production of RANKL is stimulated by a number of factors, including pro inflammatory cytokines such as IL-1, IL-6 and TNF-α, and RANKL is known to be activated in patients suffering from IBD [Franchimont, et al., Clin. Exp. Immunol. 138:491 (2004); Moschen, et al., Gut 54:479 (2005)]. Other factors, both known and unknown undoubtedly play important roles in balancing osteoclast and osteoblast activity and the disclosed invention is not limited by any specific model of regulation. The beneficial effects of the compositions and treatments disclosed here empirically improve bone status without regard to underlying theoretical molecular mechanisms.
Control of osteoblast and osteoclast activity by pro inflammatory cytokines and the association of bone disorders with IBD, suggests that interaction of the host immune system and gut microbiota may play a significant role in bone metabolism. Indeed, Sjogren and co-workers have found that germ free mice have significantly higher bone mass with a reduced number of osteoclasts relative to conventional mice raised under similar conditions [Sjogren, et al., Journal of Bone and Mineral Research 27:1357 (2012)]. Furthermore, these workers found that colonization of germ free mice with normal gut microbiota normalized bone mass and osteoclast numbers. Thus, it appears that interaction of the gut microbiota and host immune system can directly affect bone health.
Interaction between the gut microbiota and the immune system is to a large degree dependent on paracellular pathways, which are regulated by apical junctions between epithelial cells. Absent any pathologic condition, these junctions are not completely sealed but allow transport of water and solutes. These junctions are also the major route for the microbial sampling necessary to maintain immunogenic homeostasis. Loss of regulation of these junctions can result in changes in water flow, loss of coordinate solute transport and overstimulation of the immune system in response to increases in the level of microbial antigens. Such loss of regulation may be due to any number of factors, for example trauma or ulceration of intestinal epithelia may result in damage to the associated apical junctions. There is also growing evidence that certain hormonal changes in female subjects can alter regulation of apical junctions, not only in short term cycles in response to sex hormones such as estrogen and progesterone in the course of the esterous cycle, but also in response to the post menopausal cessation of such cycles [Braniste, et al., J. Physiol 587:3317 (2009)]. Similar long term alterations may occur in aging men as circulating estrogen levels decrease as a result of decreasing amounts of testosterone available for aromatization. Without being bound by theory, a method for restoring or preserving gut barrier function may improve bone health by mitigating the interaction of the gut microbiota and host immune system. Such an effect could serve to minimize unwanted stimulation of osteoclast activity and to promote osteogenesis.
In addition to modulating osteoclasts, osteoblasts actively engage in new bone formation. In this process of osteogenesis, osteoblasts lay down a protein matrix of Type I collagen and a noncollagenous protein, osteocalcin at the site of new bone synthesis. This protein matrix (the osteoid) is then mineralized with hydroxyapetite to form hard new bone [Ch. 2. In: Bone Health and Osteoporosis: A Report of the Surgeon General (2004)]. Osteocalcin, although largely localized to the osteoid, can be quantified in blood or urine samples and serves as a marker for active anabolic bone formation [Lian and Gundeberg, Clin. Orthop. Relat. Res. January(226):267 (1988)].
Currently, the only FDA-approved compound capable of stimulating new bone formation is parathyroid hormone (PTH), available commercially as Forteo® (teraperatide), a relatively expensive recombinant protein [Ch. 9. In: Bone Health and Osteoporosis: A Report of the Surgeon General (2004), p. 226]. To be effective, PTH needs to be administered by subcutaneous injection on a strict schedule to mimic the normal pulsatile action of the hormone on osteoblast differentiation [Dobnig and Turner, Endocrinology 138:4607 (1997)]. PTH may not be compatible with bisphosphonate treatment, and therefore simultaneously increasing new bone synthesis with PTH while slowing bone resorption with bisposphonate treatment is problematic [Gasser, et al., J. Musculoskeletal Neuronal Interact. 1:53 (2000)]. In addition, PTH can potentially produce serious side effects such as kidney stones, anxiety and depression, and the duration of treatment is generally limited to no more than two years due to cancer risks. Thus, there is clearly a need for simple and robust new bone anabolic agents, especially agents capable of slowing bone resorption while promoting new bone synthesis.
In addition to bone health, the interaction of the gut microbiota and the immune system is implicated in a number of other aspects of animal welfare. Modern agricultural practices induce a great deal of stress which impacts growth, production, reproduction and disease susceptibly in farm animals [see J. C. Swanson, J. Animal Sci. 73:2744 (1995) for review]. The beneficial effect of antibiotic treatment in increasing growth yield and production of farm animals has long been recognized, but is highly controversial since it may produce a reservoir of antibiotic resistant bacteria which could compromise the medical efficacy of these drugs. The need for alternatives to antibiotics to promote growth and production of farm animals is well recognized and urgent.